Method of venting controlled atmosphere furnaces



May 13, 1958 F. A. RUscIANo 2,834,699

METHOD oF VENTING coNTRoLLED ATMOSPHERE FuRNAcEs Filed May 13, 1954 3 Sheets-Sheet 2 JM- 1V PRIMARY AIR lll Il ,27 ffy/wb M NMA ATT RNEY May 13, 1958 Filed May 13, 1954 F. A. RUSCIANO METHOD 0F VENTING CONTROLLED ATMOSPHERE FURNACES 3 Sheets-Sheet 3 STACK AIR TA A PRIMARY AIR 4.95 4.90

, 34 340/ SECONDARY A|R 340 jn TEMP CONTROL 37 TERTIARY AIR 4 INVENToR. 5776 AR F.A.Rusc|ANo BY ATT RNEY United States PatentO METHOD OF VENTING CONTROLLED ATMSPHERE FURNACES Frank A. Rusciano,l New York, N. Y., assgnor to Metal` lurgical Processes Co., Newark, N. J., a corporation of New Jersey This invention relates to a furnace for the scale-free heating of metal and to a method of operation of such a furnace. It is patricularly concerned with the production in the furnace of high work temperatures and a nonoxidizing atmosphere utilizing untreated combustion products of hydrocarbon fuels and air.

The copending application of Frank A. Rusciano and Harold l. Ness, Serial No. 347,716, filed April 9, 1953, and entitled Method and Apparatus for Producing Controlled Furnace Atmospheres is directed to certain phases of the problem of attaining elevated temperatures and non-oxidizing conditions Briey, in accordance with the disclosure of the aforesaid application the non-scaling atmosphere is obtained by primary reaction of a hydrocarbon fuel and air in a proportion in which there is a substantially 50% deficiency of air for complete combustion and the high work temperature is obtained by secondary combustion of this oxygen deficient atmosphere out of contact with the work but both in heat transfer relation to the work and in exothermic relation to the primary reactions whereby to increase the reaction temperature thereof. The present invention is a continuation-impart of the aforesaid application and is directed to features which constitute my sole invention. It is particularly concerned with the maintenance of the proper pressure relationship within the work and secondary combustion chambers, in the venting of the work and secondary combustion chambers, and in the control of the secondary combustion.

One of the objects of the present invention is to prevent air leakage into the Work chamber in which the oxygen deficient atmosphere is contained with conse` quent contamination of the neutral atmosphere therein, and conversely, to prevent leakage of the oxygen decient atmosphere out of the furnace as, for instance, around doors or through door openings, with consequent burning externally of the furnace.

A further object is to provide positive and accurately controlled venting of the air deficient atmosphere from the work chamber and proper secondary combustion of the vented atmosphere in heat transfer relationship to the work chamber.

A still further object is to efficiently utilize the heat released by said secondary combustion for heating of the work. 4

Another object is to provide positively controlled venting of the secondary combustion products and to utilize such vented products recouperatively for increasing the temperature of the primary combustion.

Other objects and advantages Iwill hereinafter appear. In accordance with the present invention l utilize a furnace having a work heating chamber and a secondary combustion chamber partitioned from the work heating chamber by a wall of good heat conducting properties.

The secondary combustion chamber may be a tube or,

series of tubes extending through the work heating chamber or a compartment situated over or to one side of the Work chamber. In the specific embodiment shown it with combustion products.

2,834,599 vPatented May 1s, tsss ice consists of a chamber disposed above the work chamber. The protective atmosphere employed in the work chamber is produced by the partial combustion of fuel and air and should have a composition in which Athe sum of the CO2/CO and H2O/H2 ratios is substantially l. The double chambered furnace and manner of producing and introducing this atmosphere into the work chamber thereof do not by themselves form parts of the instant invention.

In furnaces operating at high temperatures, that is, above about 2000 F., which of necessity must be built up with high temperature ceramic brick or slabs, it isV not possible to physically seal the furnace chamber. However, when it is desired to maintain a non-oxidizing atmosphere in such a furnace, it is essentialthat leakage of air into the furnace chamber be prevented. This dictates the maintenance of a pressure within the chamber that is above atmospheric pressure whereby the leakage, if any, will be outward from the furnace chamber. Normally, such internal pressure is of the order of 0.05 to 0.1 inch of water or higher since under ordinary conditions of furnace venting it is not possible to accurately maintain lower positive pressures in the furnace chamber. These pressures are not objectionable when employing directly fired oxidizing atmospheres since moderate leakage of the atmosphere from the furnace is not of importance.

' However, when employing a non-scaling atmosphere produced directly in the heating chamber or in tubes or reaction chambers directly associated therewith, by either exothermic or endothermic reactions or both, of a hydrodue to the violent burning thereof around doors or other y leakage points and to the vigorous flaming out that occurs when a door is opened, making it difficult and hazardous to insert or remove work into or from the furnace. In the production of a 50% oxygen deficient atmosphere only about 20%-25% of the available B. t. u. content of the fuel is released, the remaining %-80% being released in any burning that occurs externally of the work chamber, thus accounting for the Violence and high temperature of this leakage combustion.

The present invention contemplates the use of a positive pressure in the work chamber to prevent air influx but, in one aspect thereof, is `directed to the accurate main.

tenance of this pressure to a value sufficiently close to atmospheric to preclude or limit to a negligible extent the external ow or leakage of the highly combustible atmos phere. This pressure control is accomplished by regulatable suction producing means in the work chamber Vents, as will fully appear hereinafter.

As previously stated, the rich 'neutral atmosphere vented from the work chamber is further combusted with additional air in a secondary combustion chamber forming a compartment of the furnace disposed in heat transfer relation to the Work chamber and the secondary air employed for this latter combustion is used, at least in part, to produce the aforesaid regulated suction condin tion. It will be understood that the amount of this sec- Vcombustion of the atmosphere. In another and preferred form the jets are fixed in such position relative to the vent ports as to produce the correct suction with a materially lesser quantity of air than is required for such secondary combustion, the correct suctional effect being obtained by control of the quantity of secondary air so supplied thereto, separate or tertiary air supply means being provided in non-venting relation to the atmosphere gases to supplement the secondary air and bring the Combustion of the atmosphere to completion in the secondary chamber.

It will be appreciated that in order to maintain a positive pressure in the work `chamber of only a few thousandths of an inch of water that a building-up of back pressure in the secondary combustion chamber must be prevented and this chamber maintained substantially at or below atmospheric pressure. A pressure approximating atmospheric may be maintained in the secondary combustion chamber by providing abnormally large vents directly open to the atmosphere. This has the disadvantage of being non-positive in control and permitting the escape `of the products of combustion before the full heating effect thereof on the work chamber has been realized. Therefore, I prefer to employ relatively small vents for the secondary combustion chamber located so as to cause a relatively long path of travel of the products in contact with the heat radiating partition or wall that separates the work chamber from the secondary com* bustion chamber and to control the pressure in this latter chamber to substantially atmospheric or sub-atmospheric by positive controllable suction producing means associated with these vents. In the embodiment disclosed air jets are employed for producing the secondary chamber vent suction. This use of pressurized air is economically justied Vsince by proper adjustment of the vent jets the amount of air required for producing the desired vent suction may be matched with that required to cool the gases to a temperature at which they may be safely passed through inexpensive metal heat exchangers suitable for preheating the air utilized for the primary combustion. This preheating of the primary air is important from other than economic considerations.

The production'of the low C02 and the high CO content gas necessary to obtain the required low CO2/CO ratios is not consistent with the development of appreciable heat, even assuming the reactions are carried to corn- Actually, with the large deficiency of air, the

pletion. reaction, if unassisted, `will not be completed, the desired CO2/CO ratio will not be obtained, and only a portion of the theoretical 20%425% B. t. u liberation Will be accomplished. If a lower air-gas ratio is employed in an attempt to obtain the desired CO2/CO ratio, the diiculty is aggravated since less heat is produced, and the re actions are further slowed down and are, therefore, less complete. With the addition of supplemental heat provided by preheated air, however, not only may the equilibrium air-fuel ratios be reacted to completion and the fuel heat of reaction obtained therefrom, but even lower air-gas ratios may be reacted substantially to completion. It is also highly desirable vthat the primary reactions be completed and stabilized prior to Contact with the work, and since the complete reaction of the gases is 'a function of both time and temperature an increase in temperature due to the use of highly heated primary air results in a faster completion of the reactions. The hotter primary atmosphere also serves as a more effective heat transfer tween the walls 14, 17, 18 and 19.

agent thus reducing the heat transfer requirement of the radiant partition between the work and secondary combustion chambers. The reaction temperature of a `50% air deficient mixture, utilizing unheated air, -is only of the order of 200G-2200o F. In high temperature furnaces where temperatures of 2300i F. to 2400o F. are required this primary atmosphere is actually below the desired work temperature, Whereas with primary air heated to form 500 F. to 1000 F. the primary atmosphere will be produced at a temperature several hundred degrees above the desired work temperature thereby greatly increasing the heating rate. v

In addition to the low positive pressure control of the work chamber, the leakage of the combustible atmosphere out of or the entrance of air into the furance through the door opening is further prevented by location of a number of the work chamber suctionally controlled vents in such position adjacent to the door as to produce a still lower pressure, which is substantially atmospheric, in the door slot or passageway. Thus a proportion of the work chamber atmosphere is drawn into the door slot and then into the vents forming a barrier for any air tending to enter the furnace chamber, and any combustion products thus formed in the door slot are drawn immediately into the slot vents. With appropriate slot venting it is possible to operate with continuously open slots without contamination of the work chamber atmosphere or burning outside of the slot opening. Thus it will be seen that the controlled venting of the work chamber and the secondary combustion chamber not only permits effective control of the furnace pressures and leakage into or out of the furnace but is also instrumental in the efficient, non-scaling high temperature 'operation of the furnace. In this connection it should be noted that the prevention of leakage of the high calorific primary atmosphere from the work chamber represents a substantial heat saving.

The invention will be best understood by reference to the accompanying drawings, in which:

Fig. l is a vertical transverse sectional view of a furnace embodying the present invention, taken on the line 1 1 of Fig. 2;

Fig. 2 is a vertical longitudinal sectional view 0f the furnace taken on the line 2-2 of Fig. l

Fig. 3 is a horizontal section of the furnace taken on the line 3--3 of Fig. 1;

Fig. 4 is a vertical sectional view taken on the line 4-4 of Fig. 2;

Fig. 5 is a detailed sectional view of an adjustable secondary air injection nozzle; and

Fig. 6 is a schematic view of the air and fuel supply conduiting of the furnace.

Reference will first be made to Figs. 1 and 2 which show a batch furnace of rectangular configuration, having a Work heating chamber 10, a secondary combustion chamber 11, and a heat exchanger 12. The work heating chamber 10 is defined by a hearth or door 13, a front vertical Wall 14 having a Work loading opening 15 normally closed by a door 16, an opposed rear wall '17 and opposite side walls 18 and 19. The secondary combustion chamber 11 is disposed directly over the work chamber 10 and is coextensive therewith, being contained be- It is provided with an arched roof 21 and is separated from the work chamber 10 by a thin arch 22 of a high temperature refractory having good heat conductivity, such as silicon carbide.

A plurality of spaced burner ports 23 extend into the work chamber 10 through the side walls 18 and 19, those extending through wall 18 being shown in full lines in Fig. 2 and the vertical and longitudinal position of the ports in Wall 19 being indicated by crosses 23. Each burner port has a high temperature refractory liner 24 and is provided with a hydrocarbon fuel by a conduit 25 ggd preheated air through a nozzle 26 fed by a conduit As previously stated, the ratio of fuel and air supplied to the ports 23 is one which has a deficiency of air for complete combustion of approximately 50%. This mixture on combustion produces products having a CO2/CO and H2O/H2 summation of approximately l vand is completely neutral to steel at the operating temperature of the furnace, which may be .up to 2350 F. to 2400'n F. The

flame temperature of such a mixtureV using cold air would be only about 2000 F. to 2200 F. but by employing air heated to 500 F. to 1000 F. the combustion temperature of the products will be of the order of 2400 F. to 2900" F. depending uponair temperature,

The work introduced through the door opening rests upon the hearth 13 and is heated by the passage of these hot gases in transit to the work chamber vent ports 28 disposed in spaced relation along the opposite side walls 18 and 19,. at the hearth level. The work is further heated by the radiant :arch 22, as will hereinafter appear. Additional vent ports 28 are provided in the front wall 14 above the work entrance passageway 15.

The vent ports 28 and 28 communicate with vertical ues 29 and 29', respectively, which in turn discharge into the secondary combustion chamber 11 through ports 31 and 31', respectively, having high temperature refractory liners 32. Each of the ports 31 and 31 is provided with an air jet nozzle 33 and 33', respectively, composed of a refractory built into the walls 18, 19 and 14, and supplied with air by conduits 34. As best shownv in Fig. 3, the ues 29V and ports 31 in side walls 18 and 19 are disposed in pairs and intermediate each pair is a port 35 having an air jet nozzle 36 supplied with air by a conduit 37. There is no ue associated with ports 35. The nozzles 33 and 36 supply suflicient air to the chamber 11 to supplement that supplied by the burner nozzles 26 to a s-uicient extent to produce complete combustion of the fuel in chamber 11. In normal operation approximately 50% of this air will be supplied by the burner or primary air nozzles 26, and 50% by the secondary air nozzles 33 and tertiary air nozzles 36. The division of this latter 50% between the nozzles 33 and 36 will depend upon the suctional requirements of the flues 29. The location of the nozzles 33 relative to their ports 31 should be such that under norm-al operatingconditions of the furnace sufticient draft will be produced in the flues 29 and 29 to maintain a very low positive pressure in chamber 10 by the use of: from 70% to 80% of the air which it is desired to supply to the chamber 11 for combustion purposes, the remaining to 30% of the air being supplied by the nozzles 36. Exact' control of the pressure in chamber 10 can then be maintained by variation in the air supplied to the nozzles 33 with compensating variations in the air supplied by nozzles 36.

It should be noted that the horizontal passages definedV by the liners 32 are relatively short and open freely into the secondary combustion :chamber 11 which in turn is maintained substantially at zero or atmospheric pressure, as will later appear. Thevvolume of cold air supplied by the air nozzle 33' is only about 1/s of that of the combnstion products inspirated through the vent 29. Under the heat of combustion a rapid expansion occurs which, if produced in the passageway 32, would create a back pressure suicient to destroy the suctional effect desired. However, due to the high velocity of the relatively heavy secondary air, no substantial expansion thereof or reaction-with the induced atmosphere from chamber 10 does occur within-this relatively short passageway and a highly effective ventmi action is obtained, the amount of gas induced through the vent 29 being several times that obtainable by natural draft alone.

As previously stated, the pressure in chamber 10 must be maintained above atmospheric to prevent leakage of air into the chamber to thereby contaminate the neutr-al reaction products but should be maintained as close as practical to atmospheric. In practice l have found that with positive pressures of from .001 to .00S inch of water inv the work ,chamber proper, a gradient decreasing substantially to zero pressure in the door slot may be readily obtained. through the slot vents 29' with a consequent freedom from leakage into or from the work chamber. A monometer may be associated with the chamber 10 to determine the pressure therein yand in the door slot 15, and manual or automatic valves associated with conduits 34 and 37 `may be used to regulate the proper flow of air to the secondary and tertiary air nozzles. However, in

general it has been found satisfactory to regulate' thei air 6 the door and then to increase the air supply until this burning.y just disappears.

The combustion' products in chamber 11 flow over the thin arch 22 thereby imparting heat to the lower work chamber. ln order to realize to the fullest extent' the heat generated in chamber 11, it is desirable to make this chamber relatively shallow, that is, to bring they arch 21 as close a's possible to the radiant arch 22 and to cause the gases to travel in. contact with arches 21 and 22 over substantial distances. However, a near atmospheric pressure in chamber 10 dictates a. still lower pressure in chamberl 11, substantially at atmospheric pressure, and this requirement is` inconsistent with a chamber of small volume since a substantial back pressure would be built up therein. Even when such a chamber is freely open to the atmosphere, as for example, by a= slot extending throughout its full length, the volume of the chamber must not be unduly limited and the path of travel of the combustion products is short. I have found, however, that by providing suicient forced venting of this chamber, the requirements of low volume and long travel path of the gases may be obtained with the required low pressure inthe chamber. As shown in Figs. 1 andl 2, the chamber 11 is vented by a series of flues 41 disposed in the rear end of the arch 21, each of these ilues having an outlet port 42 and an air injector nozzle 43 disposed so as to produce a strong suction in the vents. The' suctional eifect in these vents may be such as to produce: a ow of the order of 10 times that obtainable withv ues of equal size freely open to the atmosphere. Hence, a small number of strategically located flues will serve to maintain the desired low pressure in the secondary com= bustion chamber. In addition this air serves to cool the gaseous products from a temperature which may be as high asl 2800" F. to a safe temperature of,.say, 2000u F., for passage. through the metal alloy heat exchanger l2.l The amount of air required' to effect this cooling action will be of the order of 50% to 60% of the total air re'- quired for combusion. The heat exchanger` comprises an outer rectangular shell 44 having a horizontal partition 45 forming a passage 46 for the vented products of combustion and a passage 47 for air to be heated. The com bustion products are entered into one endof thepassage 46. and vented from the opposite end. through anelbow 4S which may feed into the furnace room vents. Air is entered by conduits 49 into one endof passage 47 and discharged at the opposite end, in counter ow to the combustion products, by conduits 27 which feed to the burner nozzles 26. Any desired amount of insulation, as shown at 51, may be applied to the outside of the heat exchanger and conduits 27 may likewise be suitably insulated. Temperatures of the primary air of fromY` 500 F. to 1.000 F. may be readily obtained by usel of a heat exchanger of proper size and design;

In Fig. 5 I have shown an adjustable venturi nozzle 33' composed of a high temperature refractory, such as silicon carbide, which extends axially of the ring 30;- of similar material disposed at the upper e'ndtof ue 29". The nozzle 33 is carried by an alloy metal cup welded to an externally threaded pipe 58. A tube 59, welded to the shell 55 of the furnace, carries a pipev ange 61" to which a plate 62 is bolted. The plate 62 has a central:

aperture in threaded engagement withy the pipe 58, and'y pipe 5SV is slotted at 63 to receive a tool for turning the; pipe toadjust the nozzle 33 relative to the port provided" by ringl 30; rl`he outer end of the pipe 58 is enclosed? in a tube 64, welded at one end to pla-te 62 and havingv a 'if-connection 65 at the opposite end, with an aircor`iduit 34. A removable plug 66 permits accessA to the pipe 58 for the adjustment of the nozzle 33' whereby to whereas with the stationary nozzle 33 of Figs. l to 3 the physical dimensions are fixed to conform to calculated values and exact adjustments made under actual operating conditions by varying the division of the secondary and tertiary air supply between the nozzles 33 and 36. The adjustable nozzle arrangement is, therefore, more flexible, and enables the proper draft to be maintained in the lues 29" by use of the full amount of air required for secondary combustion, thus eliminating the necessity for thetertiary air jets 36. 'By way of example, the nozzles 33 may be adjusted to maintain the required low pressure in the work chamber with the full volume of air required for secondary combustion when the furnace is on full tire at which time the maximum eifective heat from this secondary combustion is desired. When the furnace goes on low fire, it is not essential that maximum heating ei'ect of the secondary combustion be'obtained, and therefore the amount of air supplied to the nozzles 33 may be increased or decreased somewhat from that required to produce complete combustion in the chamber 11, as determined by the draft effect needed to maintain the work chamber at the desired low positive pressure. However,

if desired, one or more non-inspirating air inlet nozzles, i

such as 36, may be employed in conjunction with the adjustable nozzles 33 in order to obtain both the proper pressure condition in the work chamber l and the exact amount of air for complete secondary combustion in the chamber 11 on both low and high tire.

Reference will now be made to the schematic showing of Fig. 6 for supplying the fuel and the primary, secondary and tertiary air to the furnace. Air under suitable pressure, normally about 24 ounces per square inch, is supplied from a manifold 70 to the conduit 49 which extends into the heatexchanger 12 and from which it is distributed to the primary air nozzles 26 of burners 23 by conduits 27, as shown in Fig. l. Secondary air is supplied to the draft control nozzles 33 or 33 by the conduit 34, and tertiary air is supplied to the non-inspirating nozzles 36 by a conduit 37. The air employed for producing the forced venting of the secondary combustion chamber is suppliedto the nozzles 43 by conduit 71. Fuel is supplied to the primary burners 23 by conduit 25.- Each of the conduits 71, 49, 34 and 37 is provided with an electrically operated Valve 71a, 49er, 34a and 37a, re`

respective conduits, and each by-pass has a manually adjustable flow control valve 71C, 49C, 34e and 37C, respectively. The fuel supply conduit also has an adjustable electrically operated valve 25a and a by-pass 25b containing an adjustable ow control valve 2SC. All of the operating solenoids of valves 71a, 49a, 34a, 37a and 25a are connected in parallel to a source of potential through the contacts of a conventional temperature control regulatorA 72 responsive to the work chamber temperature, in such manner that all the electrically operated valves will be open when the furnace calls for high fire and closed when the furnace calls for low iire. With all the electric valves closed, the manual flow control valves 7lc, 49C, 34a` and 37o supply the air requirements, and valve 25cvthe fuel requirements for the low fire condition of the furnace. With the electric valves open the air requirements for high tire are supplied both by the by-pass valves and the main supply valves 71a, 49a, 34a and 37a, and the fuel supply by valves 25a and 25C. In the initial operation ofthe furnace the air and fuel valves are adjusted as follows: All of the electric valves are closed and a preliminary adjustment is made by opening by-pass fuel valve 25C to supply the calculated amount of fuel desired for low fire and the by-pass primary air valve49c is adjusted to provide substantially 50% aeration. With the work chamber burners firing with this non-scaling-mixture, the secondary air by-pass valve 34C is adjusted to produce and to maintain the required low positive pressure in the work chamber and the tertiary air by-pass valve 37C is adjusted to supply sutiicient additional air to the chamber 11 to obtain complete combustion of the vented primary combustion products. This is readily determined by opening valve 37C until burning occurs in the port 42 as a result of air supplied by the secondary combustion chamber vent nozzle 43 and then cutting back the air ythrough valve 37C until this burning just ceases. The air supply to nozzle 43 may then be adjusted by means of by-pass valve 71e until the desired low pressure in the secondary combustion chamber 11 is attained. With the preliminary low fire adjustments completed, the electric valves are all opened and exactly the same adjustments made for high tire through.' the manual adjusting provisions of the fuel valve 25a, primary, secondary and tertiary main air valves 49a, 34a, and 37a, respectively, and the stack vent air supply valve 71a. After the furnace has been brought up to temperature, the final setting of all of the air and fuel valves may be obtained by repeating both the low and high lire adjustments. It will be appreciated that the adjustment of the pressure of the secondary combustion chamber 11 through valves 71c and 71a will be reflected to some extent in a pressure change in the work chamber 10 and may necessitate a readjustment of the secondary and tertiary air supplies. However, when the iinal -adjustments have been completed the furnace will continue to operate properly under both high and low fire, while maintaining the required non-scaling work chamber atmosphere with complete combusion in the secondary chamber and the correct relative pressures in both the work chamber and secondary chamber.

When the adjustable secondary air nozzle 34 is employed, the same adjusting cycle is observed except that with no tertiary air required, and the conduit 37 and valves 37a and 37e will not be provided. In this case valve 34c will be adjusted to produce the requisite pressure in the work chamber and valve 34a will be adjusted to supply, in conjunction with valve 34e, the total secondary air requirements. Thereafter the nozzles 33 must be adjusted relative to their ports to obtain the proper work chamber pressure'. Several such adjustments may be necessary to place the nozzles 33' in their optimum positions.

While, as has been previously stated, the maintenance of the desired low positive pressure in the furnace and the substantially zero pressure in the door slot 15elfectively prevents entrance of the external atmosphere into the work heating chamber, even when the door is open, it will be evident that there is a zone of contact of the internal highly combustible atmosphere and the external air where combustion may occur, the products of such combustion beingdrawn into the vents 29 by the suction produced therein. This combustion is not objectionable except to the extent that the burning gases cloud the atmosphere at the door opening, making it more diflicult to observe the work within the furnace. To overcome this effect I provide a shield of completely combused gases between the internal furnace atmosphere and the external air. Referring to Figs. 2 and 4, it will be seen that a burner pipe 75 is disposedY in a slot 76 in the base of the door slot 1S. The pipe 75 has a series of burner ports 77 at which a completely combustible mixture, supplied by a conduit 78, is burned. It will be understood that combustion is substantially completed in the slot 76, the burned products passing upwardly across the door opening to the vents 29 and acting as a curtain between the furnace gases and the external air. Since this curtain is composed of completely burned gases, it has substantially no reactiontwith the air and has only a slight amount of reaction with the furnace gases, thereby effectively maintaining a clear atmosphere acrossthe door opening.

It will be understood that the invention is applicable both to batch and continuous furnaces and to furnaces having normally closed doors, as shown, or normally open slots. It is also not restricted to the particular details shown and described but is susceptible to numerous variations and embodiments without departing from the essential attributes thereof, and I contemplate all such variations and embodiments as coming within the scope ot the appended claims.

What I claim is:

l. The method of producing scale-free heating of metal at high temperatures in a furnace having a work chamber and a second chamber in heat transfer relation to said work chamber which comprises thermally reacting a mixture of fuel and air having a large deficiency of air for complete combustion to produce hot reaction products, continuously supplying a suicient volume of the unmodified hot reaction products of said such reaction to the Work chamber to heat the same and to maintain a positive pressure of said products in said chamber, continuously conveying the products from said work chamber into said second chamber, adding air to said second chamber in an amount substantially equal to said deficiency, utilizing said added air, at least in part, to covey said products from said work chamber to said second chamber at a rate which will maintain a positive pressure of from .001 to .008 inch of water in said work chamber, burning said products to completion with said air in the second chamber, to produce secondary products of complete combustion therein in heat imparting relation to said work chamber and venting said secondary products of combustion from said second chamber at a rate which will maintain a pressure in said second chamber below that in the work chamber.

2. The method of producing scale-free lheating of metal at high temperatures in a furnace having a work chamber and a second chamber in heat transfer relation to said work chamber Which comprises thermally reacting a mixture of fuel and air having a large deficiency of air for complete combustion to produce hot reaction products, continuously supplying a su'icient volume of the unmodied hot reaction products of said such reaction to the work chamber to heat the same and to maintain a positive pressure of said products in said chamber continuously conveying the products from said work chamber into said second chamber, entering an amount of secondary air into said second chamber which is less than said deficiency, utilizing said secondary air to convey said products to said second charnber at a rate which will maintain a positive pressure in said Work chamber below 0.008 inch of water, adding tertiary air to said second chamber in an amount which with said secondary air will substantially equal said deciency, and burning said products to completion with said secondary and tertiary air in said chamber to produce secondary products of `complete combustion therein in heat imparting relation to said work chamber.

3. The method of producing scale-free heating of metal at high temperatures in a furnace having a work chamber and a second chamber in heat transfer relation to said work chamber which comprises thermally reacting a mixture of fuel and air having a large deficiency of air for complete combustion to produce hot reaction products, continuously supplying a sufficient volume of the unmodified hot reaction products of said such reaction to the work chamber to heat the same and to maintain a positive pressure of said products in said chamber, continuously conveying the products from said work chamber into said second chamber, entering an amount of secondary air into said second chamber which less than said deficiency, utilizing said secondary air to convey said products to said second chamber at a rate which will maintain a positive pressure in said work chamber below 0.008 inch of Water, adding tertiary air to said second chamber in an amount which with said secondary air will substantially equal said deficiency, burning said products to completion With said secondary and tertiary air in said chamber to produce secondary products of complete combustion therein in heat imparting relation to said work chamber, and venting said secondary products of combustion from said second chamber at a rate which will maintain a pressure in said second chamber below that in the Work chamber.

4. In the operation of a furnace having a work chamber continuously supplied with a highly combustible gas in sutlicient volume to maintain a positive pressure therein, a work entering passageway of lesser cross-sectional area than said Work chamber having an opening to the outer atmosphere and a combustion chamber in heat transfer relation to said work chamber, the method of preventing the burning of said gas externally of said furnace adjacent said opening and utilizing the available heat of said gas which comprises venting the gas from said work chamber, at least in part, from said passageway, conveying said vented gas to said combustion chamber, adding air to said vented gas to produce a combustible mixture within said combustion chamber, and utilizing said air to produce a rate of venting of such gas from said passageway which willmaintain a positive pressure of said gas in said work chamber and a pressure of said gas in said passageway which is substantially atmospheric.

5. In the operation of a furnace having a work chamber continuously supplied with a highly combustible gas in suicient volume to maintain a positive pressure therein, a work entering passageway of lesser cross-sectional area than said work chamber having an opening to the outer atmosphere and a combustion chamber in heat transfer relation to said work chamber, the method of preventing the burning of said gas externally of said furnace adjacent said opening and utilizing the available heat of said gas which comprises venting the gas from said work chamber, at least in part, from said passageway, conveying said vented gas to said combustion chamber, introducing combustion supporting air into said combustion chamber and utilizing said air to produce an induced suctional elfect on said conveyed gas to increase the rate of venting of said gas from said passageway and controlling said suctional effect to maintain a positive pressure of said gas in said work chamber and a pressure of said gas in said passageway which is substantially atmospheric.

References Cited in the file of this patent UNITED STATES PATENTS 1,752,433 Hortvet Apr. 1, 1930 1,849,714 Hayes Mar. 15, 1932 1,921,763 MacDougall Aug. 8, 1933 2,233,474 Dreffein Mar. 4, 1941 2,275,106 Hayes Mar. 3, 1942 2,402,013 Billeter et al. June 11, 1946 2,430,191 Schrumn Nov. 4, 1947 2,504,320 Gamble Apr. 18, 1950 2,529,155 Hasselhorn Nov. 7, 1950 

1. METHOD OF PRODUCING SCALE-FREE HEATING OF METAL AT HIGH TEMPERATURES IN A FURNACE HAVING A WORK CHAMBER AND A SECOND CHAMBER IN HEAT TRANSFER RELATION TO SAID WORK CHAMBER WHICH COMPRISES THERMALLY REACTING A MIXTURE OF FUEL AND AIR HAVING A LARGE DEFICIENCY OF AIR FOR COMPLETE COMBUSTION TO PRODUCE HOT REACTION PRODUCTS, CONTINUOUSLY SUPPLYING A SUFFICIENT VOLUME OF THE UNMODIFIED HOT REACTION PRODUCTS OF SAID SUCH REACTION TO THE WORK CHAMBER TO HEAT THE SAME AND TO MAINTAIN A POSITIVE PRESSURE OF SAID PRODUCTS IN SAID CHAMBER, CONTINUOUSLY CONVEYING THE PRODUCTS IN SAID WORK CHAMBER INTO SAID SECOND CHAMBER, ADDING AIR TO SAID SECOND CHAMBER IN AN AMOUNT SUBSTANTIALLY EQUAL TO SAID DEFICIENCY, UTILIZING SAID ADDED AIR, AT LEAST IN PART, TO COVEY SAID PRODUCTS FROM SAID WORK CHAMBER TO SAID SECOND CHAMBER AT A RATE WHICH WILL MAINTAIN A POSITIVE PRESSURE OF FROM .001 TO .008 INCH OF WATER IN SAID WORK CHAMBER, BURNING SAID PRODUCTS TO COMPLETION JWITH SAID AIR IN THE SECOND CHAMBER, TO PRODUCE SECONDARY PRODUCTS OF COMPLETE COMBUSTION THEREIN IN HEAT IMPARTING RELATION TO SAID WORK CHAMBER AND VENTING SAID SECONDARY PRODUCTS OF CUMBUSTION FROM SAID SECOND CHAMBER AT A RATE WHICH WILL MAINTAIN A PRESSURE IN SAID SECOND CHAMBER BELOW THAT IN THE WORK CHAMBER. 